Transformation of doped graphite into cluster-encapsulated fullerene cages.

An ultimate goal in carbon nanoscience is to decipher formation mechanisms of highly ordered systems. Here, we disclose chemical processes that result in formation of high-symmetry clusterfullerenes, which attract interest for use in applications that span biomedicine to molecular electronics. The conversion of doped graphite into a C80 cage is shown to occur through bottom-up self-assembly reactions. Unlike conventional forms of fullerene, the iconic Buckminsterfullerene cage, I h-C60, is entirely avoided in the bottom-up formation mechanism to afford synthesis of group 3-based metallic nitride clusterfullerenes. The effects of structural motifs and cluster-cage interactions on formation of compounds in the solvent-extractable C70-C100 region are determined by in situ studies of defined clusterfullerenes under typical synthetic conditions. This work establishes the molecular origin and mechanism that underlie formation of unique carbon cage materials, which may be used as a benchmark to guide future nanocarbon explorations.

La3N@C92: An Endohedral Metallofullerene Governed by Kinetic Factors?

Different structures have been proposed so far for the C92 isomer that encapsulates M3N (M = La, Ce, Pr). We show here that the electrochemical properties of the predicted most abundant (thermodynamic) isomer for La3N@C92 does not agree with experiment. After a systematic search within the huge number of possible C92 isomers, we propose other candidates with larger electrochemical gaps for La3N@C92 before its structure could be finally determined by X-ray crystallography. We do not discard that the thermodynamic isomer could be detected in future experiments though.

Small endohedral metallofullerenes: exploration of the structure and growth mechanism in the Ti@C2n (2n = 26-50) family.

The formation of the smallest fullerene, C28, was recently reported using gas phase experiments combined with high-resolution FT-ICR mass spectrometry. An internally located group IV metal stabilizes the highly strained non-IPR C28 cage by charge transfer (IPR = isolated pentagon rule). Ti@C44 also appeared as a prominent peak in the mass spectra, and U@C28 was demonstrated to form by a bottom-up growth mechanism. We report here a computational analysis using standard DFT calculations and Car-Parrinello MD simulations for the family of the titled compounds, aiming to identify the optimal cage for each endohedral fullerene and to unravel key aspects of the intriguing growth mechanisms of fullerenes. We show that all the optimal isomers from C26 to C50 are linked by a simple C2 insertion, with the exception of a few carbon cages that require an additional C2 rearrangement. The ingestion of a C2 unit is always an exergonic/exothermic process that can occur through a rather simple mechanism, with the most energetically demanding step corresponding to the closure of the carbon cage. The large formation abundance observed in mass spectra for Ti@C28 and Ti@C44 can be explained by the special electronic properties of these cages and their higher relative stabilities with respect to C2 reactivity. We further verify that extrusion of C atoms from an already closed fullerene is much more energetically demanding than forming the fullerene by a bottom-up mechanism. Independent of the formation mechanism, the present investigations strongly support that, among all the possible isomers, the most stable, smaller non-IPR carbon cages are formed, a conclusion that is also valid for medium and large cages.

Bottom-up formation of endohedral mono-metallofullerenes is directed by charge transfer.

An understanding of chemical formation mechanisms is essential to achieve effective yields and targeted products. One of the most challenging endeavors is synthesis of molecular nanocarbon. Endohedral metallofullerenes are of particular interest because of their unique properties that offer promise in a variety of applications. Nevertheless, the mechanism of formation from metal-doped graphite has largely eluded experimental study, because harsh synthetic methods are required to obtain them. Here we report bottom-up formation of mono-metallofullerenes under core synthesis conditions. Charge transfer is a principal factor that guides formation, discovered by study of metallofullerene formation with virtually all available elements of the periodic table. These results could enable production strategies that overcome long-standing problems that hinder current and future applications of metallofullerenes.

Relevance of thermal effects in the formation of endohedral metallofullerenes: the case of Gd3N@C(s)(39663)-C82 and other related systems.

Thermal contributions to the free energy have to be taken into account to rationalize the formation of Gd(3)N@C(s)(39663)-C(82), a nitride endohedral metallofullerene that shows a carbon cage with two fused pentagons which is not predicted to have the lowest electronic energy among the isomers of C(82). The lower symmetry and the larger number of pyracylene units of C(s)(39663)-C(82) with respect to the cage in the lowest-energy metallofullerene, C(2v)(39705)-C(82), favor its formation at high temperatures, as seen for other similar cage isomers that encapsulate metal clusters within the C(80) and C(82) families. These cages, which share common motifs with the prototypical I(h)(7)-C(80), are all related by C(2) insertions/extrusions and Stone-Wales transformations.

The smallest stable fullerene, M@C28 (m = Ti, Zr, U): stabilization and growth from carbon vapor.

The smallest fullerene to form in condensing carbon vapor has received considerable interest since the discovery of Buckminsterfullerene, C(60). Smaller fullerenes remain a largely unexplored class of all-carbon molecules that are predicted to exhibit fascinating properties due to the large degree of curvature and resulting highly pyramidalized carbon atoms in their structures. However, that curvature also renders the smallest fullerenes highly reactive, making them difficult to detect experimentally. Gas-phase attempts to investigate the smallest fullerene by stabilization through cage encapsulation of a metal have been hindered by the complexity of mass spectra that result from vaporization experiments which include non-fullerene clusters, empty cages, and metallofullerenes. We use high-resolution FT-ICR mass spectrometry to overcome that problem and investigate formation of the smallest fullerene by use of a pulsed laser vaporization cluster source. Here, we report that the C(28) fullerene stabilized by encapsulation with an appropriate metal forms directly from carbon vapor as the smallest fullerene under our conditions. Its stabilization is investigated, and we show that M@C(28) is formed by a bottom-up growth mechanism and is a precursor to larger metallofullerenes. In fact, it appears that the encapsulating metal species may catalyze or nucleate endohedral fullerene formation.

Sc2S@C(s)(10528)-C72: a dimetallic sulfide endohedral fullerene with a non isolated pentagon rule cage.

A non isolated pentagon rule metallic sulfide clusterfullerene, Sc(2)S@C(s)(10528)-C(72), has been isolated from a raw mixture of Sc(2)S@C(2n) (n = 35-50) obtained by arc-discharging graphite rods packed with Sc(2)O(3) and graphite powder under an atmosphere of SO(2) and helium. Multistage HPLC methods were utilized to isolate and purify the Sc(2)S@C(72). The purified Sc(2)S@C(s)(10528)-C(72) was characterized by mass spectrometry, UV-vis-NIR absorption spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction. The crystallographic analysis unambiguously elucidated that the C(72) fullerene cage violates the isolated pentagon rule, and the cage symmetry was assigned to C(s)(10528)-C(72). The electrochemical behavior of Sc(2)S@C(s)(10528)-C(72) shows a major difference from those of Sc(2)S@C(s)(6)-C(82) and Sc(2)S@C(3v)(8)-C(82) as well as the other metallic clusterfullerenes. Computational studies show that the Sc(2)S cluster transfers four electrons to the C(72) cage and C(s)(10528)-C(72) is the most stable cage isomer for both empty C(72)(4-) and Sc(2)S@C(72), among the many possibilities. The structural differences between the reported fullerenes with C(72) cages are discussed, and it is concluded that both the transfer of four electrons to the cage and the geometrical requirements of the encaged Sc(2)S cluster play important roles in the stabilization of the C(s)(10528)-C(72) cage.